Ion chromatography (IC) has become a widely used analytical technique for the determination of anionic and cationic analytes in various sample matrices since it was introduced in 1975. Ion chromatography today is performed in a number of separation and detection modes. Ion chromatography with suppressed conductivity detection is the most widely practiced form of the technique. In suppressed conductivity detection, an eluent suppression device, termed a suppressor, converts the eluent into a weakly conducting form and enhances the conductance of target analytes. The original suppressors were columns packed with ion-exchange resins in appropriate ionic forms. Those packed-bed suppressors had a relatively large dead volume and required off-line chemical regeneration. To overcome this problem, suppressors based on ion-exchange fibers and other membranes were developed. These suppressors can be continuously regenerated using either acid or base regenerant solutions.
One disadvantage associated with the original membrane suppressors was that an external source of either acid or base regenerant solution typically was used to generate the suppressor continuously. Over the years, various designs of electrolytically-regenerated membrane suppressors as described in U.S. Pat. Nos. 4,999,098, 5,248,426, 5,352,360, and 6,325,976 have been developed to overcome the limitations associated with the chemically-regenerated membrane suppressors. The electrolytic suppressors offer several advantages in ion chromatography. They provide continuous and simultaneous suppression of eluents, regeneration of the suppression medium, and sufficient suppression capacity for common IC applications. They are easy to operate because the suppressed eluent or water can be used to create regenerant ions electrolytically. Thus, there is no need to prepare regenerant solutions off-line. Also, the suppressors are compatible with gradient separations. They have very low suppression zone volume, which makes it possible to achieve separations with high chromatographic efficiency.
In ion chromatography, dilute solutions of acids, bases, or salts are commonly used as chromatographic eluents. Traditionally, these eluents are prepared off-line by dilution with reagent-grade chemicals. Off-line preparation of chromatographic eluents can be tedious and prone to operator errors, and often introduces contaminants. For example, dilute NaOH solutions, widely used as eluents in the ion chromatographic separation of anions, are easily contaminated by carbonate. The preparation of carbonate-free NaOH eluents is difficult because carbonate can be introduced as an impurity from the reagents or by adsorption of carbon dioxide from air. The presence of carbonate in NaOH eluents can compromise the performance of an ion chromatographic method, and can cause an undesirable chromatographic baseline drift during the hydroxide gradient and even irreproducible retention times of target analytes. In recent years, several approaches that utilize the electrolysis of water and charge-selective electromigration of ions through ion-exchange media have been investigated by researchers to purify or generate high-purity ion chromatographic eluents. U.S. Pat. Nos. 6,036,921, 6,225,129, 6,316,271, 6,316,270, 6,315,954, and 6,682,701 describe electrolytic devices that can be used to generate high purity acid and base solutions by using water as the carrier. Using these devices, high purity, contaminant-free acid or base solutions are automatically generated on-line for use as eluents in chromatographic separations. These devices simplify gradient separations that can now be performed using electrical current gradients with minimal delay instead of using a conventional mechanical gradient pump.
The combined use of the electrolytic eluent generator and suppressor has significantly changed the routine operation of ion chromatographic methods and permits the performance various ion chromatographic separations using only deionized water as the mobile phase. The use of these electrolytic devices results in significant improvements in the performance of ion chromatography methods by allowing minimal baseline shifts during the gradients, greater retention time reproducibility, lower detection backgrounds, and lower detection limits for target analytes.
Recently, capillary high performance liquid chromatography using separation columns with internal diameters of 1 mm or smaller has gained increasing popularity as an analytical separation tool because of the advantages associated with the miniaturization of separation processes. The typical separation columns in ion chromatography have column internal diameters ranging 2 mm to 4 mm and are operated in flow rate ranging from 0.2 to 3 mL/min. The practice of ion chromatography in the capillary format (i.e., using small bore columns with internal diameters of about 1 mm or smaller) potentially has a number of advantages for analysis of ionic analytes. The use of capillary separation column can improve the separation efficiency and/or speed. Separation processes in the capillary format require much smaller amount of sample and thus offer improved compatibility with applications where amount of sample is limited. Capillary ion chromatography system typically operates at 1 to 20 μL/min and thus the amount of eluent consumed is very small. Capillary ion chromatography has improved capability for continuous operation with minimal intervention and thus minimizes problems associated with system start-up and shutdown. The operation of capillary ion chromatography at low flow rates improves the system compatibility with mass spectrometer. In addition, the practice of ion chromatography in the capillary format opens the door for the possibilities of offering new selectivity for difficult applications using new columns packed with more exotic and difficult-to-make stationary phases.
When compared to high performance liquid chromatography, ion chromatography has progressed slower in the area of miniaturization of the dimension of the separation process. A limited number of studies have been reported so far in the area of capillary ion chromatography using suppressed conductivity detection. In 1983, Rokushika and co-workers reported the development of a capillary ion chromatography system using suppressed conductivity detection (J. Chromatography, 260 (1983) 81-88). In their study, an anion exchange capillary column was prepared by packing a surface-agglomerated anion exchange resin in a fused silica capillary with an internal diameter of 190 μm. The suppressor was fabricated using a Nafion® hollow fiber tubing and was regenerated chemically using an external solution of 0.05 M deodecylbenzenesulfonic acid. Separations of inorganic anions and carboxylic acids were disclosed. In 1997, Dasgupta and coworker reported the implementation of a capillary ion chromatography system using an on-line high pressure electrolytic sodium hydroxide eluent generator (Anal. Chem., 29 (1997) 1385-1391). In their system, deionized water was used as the carrier for electrolytic generation of sodium hydroxide eluents at 2 μL/min typically, a capillary column packed with anion exchanger was used as the separation column, and a suppressor prepared using Nafion® tubing and regenerated chemically using a solution of sulfuric acid was used. Both isocratic and gradient separations of inorganic and organic anions were disclosed. In 2001, Pyo and Kim reported their work on the development of capillary ion chromatography using open tubular columns and suppressed conductivity detection (J. Korean Chem. Soc., 2001, Vol. 45, No. 3). Open tubular capillary columns coated with DMEOHA latex particles were used as separation columns. The suppressor was fabricated using a Nafion® hollow fiber tubing and regenerated chemically using an external acid solution.
In the publications discussed above, capillary ion chromatography with suppressed conductivity detection was performed using suppressors made of ion-exchange capillary tubing. These publications disclose chemical regeneration using an external dilution acid solution. The dead volume of this type of suppressors can be minimized so that they are compatible with the capillary separation columns. However, these publications disclosed the use of chemical regenerant, adding costs of dispensing and disposing of the chemical regenerant, resulting in potential leakage of the chemical regenerant across the ion-exchange membrane into the eluent, which raises the conductivity detection background and affects negatively the sensitivity of some analytes. There is a need for a capillary ion chromatography system with an easy-to use, rugged, and reliable capillary suppressor.